TI BASIC was built into the 99/4 and 99/4A, and was remarkably easy to learn. Users could access functions through this simple language that on other computers would only be available to those versed in the machines' assembly language code. Text, full screen graphics, logic operations, and recording to tape (or any other file system - diskettes, RS-232 interface, RAM disks) could all be accomplished using TI BASIC. TI also produced an Extended BASIC cartridge that greatly enhanced the functionality accessible to BASIC users.

Every line of TI BASIC can only contain one statement. The possibility to put several statements in a line with :: and also the necessary PEEK and LOAD calls to use assembly language routines became available in TI Extended BASIC. Note that there is no POKE statement in TI BASIC. Loading and executing machine code requires a 32K memory expansion and the TI Editor/Assembler cartridge, which then gives access CALL LOAD. The 256 Bytes of RAMdirectly accessible by the CPU was mostly consumed by the BASIC interpreter. TI BASIC stored and ran programs from the 16K video RAM, and, unlike TI Extended BASIC, could not take advantage of the 32K memory expansion.

One serious shortcoming of TI BASIC was that, as an interpreted rather than compiled language, it was inherently slow. TI BASIC was a double interpreted language, as the BASIC interpreter was itself written in a mid-layer interpreted byte-code language known as Graphics Programming Language (GPL), which was unique to the TI Home Computer.